Periodic structures are formed during development in diverse biological systems. The main objective of this proposal is to examine the periodic formation of lateral roots (LRs) in plants, within the framework established by the 'clock-and-wavefront'model for the periodic formation of somites in vertebrates. The model for somitogenesis proposes that a traveling molecular oscillator, the segmentation clock, sets the period of somite formation. The clock signal is translated into a spatial pattern of periodic segments by interaction with two opposing signaling gradients at the 'wavefront', which is where cells become competent to form somites in response to the clock signal. This 'clock-and-wavefront'mechanism accounts for the periodic serial formation of somites along the vertebrate axis. A traveling molecular oscillator has also been identified in roots and disruption of it alters LR formation. Moreover, one hormone gradient is already predicted in the root and we have evidence implicating a second gradient. We propose that the 'clock-and-wavefront'mechanism may be a common regulatory strategy in the formation of periodic structures in diverse multicellular eukaryotes. To test our hypothesis that a 'clock-and-wavefront'mechanism is operating in LR development, three specific aims will be addressed. The behavior of the oscillation will be examined and mathematically modeled. The hypothesis that a retinoid gradient operates in the root will be tested and the interaction of the gradient with the oscillation will be examined. Additionally, natural variation in LR formation will be used to identify genes involved in this process. The primary method in analyzing the oscillation and the gradient in LR formation will be real-time imaging of luciferase reporters in transgenic plants. The response of the oscillation and subsequent competency to form a LR will be examined upon manipulation of the gradients by chemical and molecular biological methods. The results of these experiments will be used to mathematically model the behavior of the oscillation and its interaction with the gradient. Finally, natural variation in LR formation will be used to identify genes involved in this process by association studies and analysis of quantitative trait loci. These studies constitute a novel way of analyzing LR development and will reveal insight into the fundamental question of how LR periodicity is established and maintained.